The use of mucosal lymphoid tissue is critical in emerging HIV research as studies have demonstrated the pivotal importance of lymphoid tissue has a major reservoir of replicative HIV-1. Recent studies have begun to address the differential rates of CD4+ T cell depletion in lymphoid tissue and peripheral blood with attempts to compare viral burden in lymphoid tissue and plasma. These studies have immediate clinical implications regarding continuation of therapy. The gut-associated lymphoid tissue (GALT) represents the body's major lymphoid organ, most of which is directly, quickly and safely accessible by endoscopic biopsy. In addition, the mucosal lymphoid populations are among the first lines of immunological defense, preceding the second phase of more organized immunological processing centers of lymph nodes and tonsillar tissue. Importantly this mucosal tissue is rapidly replace and can be evaluated frequently, in contrast to lymph nodes which are finite in number limiting their use in monitoring tissue response. The new Mucosal Immunology Core will aim to provide interested researchers with clinically well-characterized tissue samples from gastrointestinal mucosal sites for study. From a clinical perspective, this will involve consultation in study design, recruitment of appropriate patient cohorts, assistance in submission of individualized human subjects protection applications. The laboratory component will provide optimized techniques in providing fresh and frozen samples of HIV seropositive subjects and seronegative controls. Samples are easily collected from patients in clinical trials which would allow assessment of viral impact, for example, before and after exposure to new medication regimens. Site-specific biopsies can be obtained for investigations of esophageal, stomach, duodenal, ileal, colonic and rectal tissue. Single cell collections, such as mononuclear mucosal preparations for flow cytometry, or immunoglobulin secretions from saliva or rectal mucosa can be collected on a prospective basis for interested investigators. Other mucosal sites (pharyngeal, pulmonary, vaginal, urinary system) and other patient populations (including women and minorities) will be available through a network of supportive clinicians.